Truckers have been doing that for years, I never done it as used engine oil has a possibility of fine particles of metal in it. But it can't be terribly bad, some of those old diesels have several million miles on em.
We used to dump all of our drain oil into our Diesel UST at time of re-fill. The fuel out of the pump had a grey tint to it but no problems. We were running DT466's and one 3208. I would do it now with my Cummins if I drove enough to make it worthwhile. I wouldn't be afraid to try it with a common rail too.